The present invention relates to intrusion detectors such as are used for protecting a specific confined area against intrusion by unauthorised persons.
A number of intrusion detection devices are known. One type includes an electrical switch which is attached to a wire fence enclosing the area to be protected, such that when the wire is pulled apart or cut in order to gain entry into the protected area, the change in tensioning of the wire actuates the electrical switch to provide a signal or alarm. One of the problems in such an intrusion detector device is to prevent false alarms arising from the wire being tensioned by temperature changes or the wind, so that the intrusion detector is responsive only to actual pulling or cutting the wire.
A number of techniques have been proposed for solving the above problem. One technique, as illustrated in U.S. Pat. No. 3,634,638, includes a switch construction which distinguishes between quick movements on the one hand as caused by pulling or cutting the wire, and slow movements on the other hand as caused by temperature changes or the wind. Another proposed arrangement, as illustrated in U.S. Pat. No. 3,243,797, senses the rate of change of the wire tension to determine whether the change is due to intrusion or to temperature, and if the change is gradual because of a temperature change, then a motor rebalancing system is actuated to compensate for the gradual temperature change.
One object of the present invention is to provide a new and improved intrusion detector device for use with a tensioned wire, which device can be conveniently preset for a specific operating point.
An additional object of the present invention is to provide an intrusion detector device having improved means for discriminating between a tensioning of the wire caused by an intrusion, and a tensioning of the wire caused by temperature or wind changes, so that the detector is actuated only upon the occurrence of the former and not upon the occurrence of the latter. Another object is to provide an improved intrusion detector based on the use of a magnetic reed switch.
According to the present invention, there is provided an intrusion detector comprising a housing attachable to a fixed support and an electrical switch fixed within the housing. The housing is formed with a longitudinal bore extending at least partly therethrough, the switch actuator being movable within the longitudinal bore to actuate the switch. The actuator is attachable to a wire tensioned between it and another fixed support so as to actuate the switch upon stretching or cutting the wire. Disposed within the bore is a spring having one end engaging the movable actuator to bias same in the direction opposite to the tensioned wire. An adjustable closure member closes one end of the bore and engages the opposite end of the spring to preset the initial non-actuating position of the actuator with respect to the switch.
In one described embodiment, the tensioned wire is a stretchable wire tensioned between two fixed supports, the movable switch actuator being attached to the tensioned wire at an intermediate point thereof such that the stretchability of the span of wire between one of the wire supports and the movable switch actuator is substantially equal to that of the span of wire between the other wire support and the movable actuator, whereby the switch is actuated only upon the differential tensioning of the two wire spans such as caused by pulling or cutting the wire and not upon the equal and opposite tensioning of the two wire spans such as caused by temperature changes or the wind.
In another described embodiment, the tensioned wire includes only one span, one end of the wire being connected to the fixed wire support and the opposite end of the wire being connected to the movable switch actuator.
Preferably, the electrical switch is a reed switch, the switch actuator including a permanent magnet for actuating the switch when moved a predetermined amount.
The preferred embodiment of the invention described below, which includes the differential tensioning means, thus provides a very simple and effective arrangement for discriminating between the tensioning of the wire caused by an intrusion, and a tensioning of the wire caused by temperature or wind changes, and actuates the detector only upon the occurrence of the former and not upon the occurrence of the latter.
However, features of the invention can advantageously be used even without the differential tensioning arrangement, as will be apparent from the second embodiment of the invention described below.